2 Answers
2

In Unix all normal files are Hardlinks. Hardlinks in a Unix (and most (all?) ) filesystems are references to an to what's called an inode. The inode has a reference counter, when you have one "link" to the file (which is the normal modus operandi) the counter is 1. When you create a second, third, fourth, etc link, the counter is incremented (increased ) each time by one. When you delete (rm) a link the counter is decremented ( reduced ) by one. If the link counter reaches 0 the filesystem removes the inode and marks the space as available for use.

In short, as long as you do not delete the last link the file will remain.

Edit: The file will remain even if the last link is removed. This is one of the ways to ensure security of data contained in a file is not accessible to any other process. Removing the data from the filesystem completely is done only if the data has 0 links to it as given in its metadata and is not being used by any process.

This IMHO is by far the easiest way to understand hard-links (and its difference from softlinks).

And yes, calling unlink() to delete a file puzzled me to no end when I started playing with C programming on MS-DOS some time back in the first half of the 1990s. :)
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Michael KjörlingOct 8 '12 at 11:13

Testing was easier than I thought: I created a text file, then hard linked to it. Deleting the hard link does not delete the file it is hardlinked to and the file that was linked to remains where it is.